Audio Tone Teleprotection is a method of communicating binary states from one substation to another over audio telecommunications links. These binary states are used, for example, to control power line circuit breakers and generators. This practice has existed for over 30 years and has principally been applied over entirely analog systems such as Frequency Division Multiplexers, Analog Microwave, and Direct Analog Fiber.
When a signal occurs in any of the above media, noise occurs at the input of the teleprotection and circuitry inside the teleprotection is designed to prevent a “misoperation”. A misoperation is a false state on one of the binary outputs. The result of a false output can be disastrous, e.g. blackouts, fires, and/or equipment damage. It is important to prevent these misoperations in the event of loss of signal, since systems like digital microwave are frequently susceptible to loss of signal, usually due to adverse weather conditions.
With the recent proliferation of telecommunications has come the practice of applying audio teleprotection over Time Division Multiplexed (TDM) digital communications networks. Under failure conditions, different noise patterns occur at the input of the teleprotection. These noise patterns can very closely mimic valid signals indicating incorrect states, possibly causing misoperation of the teleprotection equipment.
The most common form of TDM used in teleprotection is known as T1 or DS-1. T1 is a format defined by the Bell system for digital transmission of analog signals. It is designed to carry 24 audio channels over a 1.544 Mb/s serial digital link, as illustrated in FIG. 1. As shown in FIG. 2, one frame has 24 8-bit words (192 bits) plus 1 framing bit, so one out of every 193 bits is used to create a framing pattern. T1 frames are transmitted at the rate of 8000 per second, so, with 193 bits per frame, the bit rate is 1.544 megabits per second. This framing pattern repeats every 1.5 or 3.0 ms, depending on whether Superframe or Extended Superframe is employed. Superframe is a 1.5 ms frame consisting of 12 consecutive T1 frames. Since standard framing practice is to look for at least 2 bad framing patterns before declaring a loss of frame, the process can take at least 3 ms. Extended Superframe is made up of 24 frames and can take 6 milliseconds or more to detect a loss of frame.
With standard telephone grade multiplexers, a noisy signal is output from the voice channels for up to 2 seconds after a complete loss of signal.
It is among the objects of the present invention to provide a method and apparatus which overcomes disadvantages of existing approaches such as those described, and to prevent misoperations of teleprotection systems in the event of TDM signal loss. It is among the further objects of the invention to achieve this in a way that is compatible with existing teleprotection design, due to the large installed base of teleprotection equipment.
Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.